Low supply voltage temperature compensated reference voltage generator and method

ABSTRACT

A reference voltage generator uses a conventional forward junction voltage generating device and a conventional thermal generator to generate a thermal voltage. The forward junction voltage and the thermal voltages have respective thermal sensitivities that act oppositely to each other so that, when the forward junction voltage is combined with the thermal voltage to produce a reference voltage, the reference voltage is substantially insensitive to temperature. The forward junction voltage and the thermal voltage are combined to produce the reference voltage in a manner that avoids generating any voltage having a magnitude that is greater than the magnitude of the sum of the forward voltage and the thermal voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 10/932,480, filed Sep. 1, 2004, now U.S. Pat. No. 7,116,588.

TECHNICAL FIELD

This invention relates to voltage reference circuits, and, more particularly, to a temperature compensated diode voltage circuit that can generate a temperature compensated voltage using a relatively low supply voltage.

BACKGROUND OF THE INVENTION

A variety of electronic devices and circuits must be supplied with a voltage having a magnitude that is precisely controlled using a voltage regulator, charge pump or other voltage source. The magnitude of the voltage supplied by the voltage regulator, charge pump or other source is often set by the magnitude of a reference voltage. Various reference voltage generators are well known in the art.

A common reference voltage generator is a diode voltage generator, a typical example of which is shown in FIG. 1. The diode voltage generator 10 includes a reference current source 14 directing a reference current I_(R) through a bipolar PNP transistor 20 having its base and collector interconnected in a diode configuration. A PN junction diode forward voltage V_(EB) is produced at the emitter of the transistor 20. Of course, other designs for a diode voltage generator can be used, such as one having a bipolar NPN transistor.

The current-voltage characteristics of the diode voltage generator 10 are shown in FIG. 2. As shown in FIG. 2, as the current flowing through the transistor 20 increases from zero, the emitter-base voltage V_(EB) increases quite dramatically until the “knee” 22 of the curve is reached. Thereafter as the current increases further, the emitter-base voltage V_(EB) is fairly constant despite large changes in the magnitude of the current. The current source 14 maintains the reference current I_(R) at a magnitude that is greater than the magnitude of the current at the knee 22. As a result, the diode reference voltage V_(EB) is maintained at a relatively constant value despite slight fluctuations in the magnitude of the reference current I_(R). A typical value for the diode reference voltage V_(BG) is 0.65 volts, and the diode voltage generator 10 is able to maintain that voltage to within a few millivolts.

Diode voltage generators are often exposed to environments in which the temperature can vary widely, and yet it is important to maintain the reference voltage constant despite these temperature variations. Unfortunately, although the diode reference voltage V_(EB) is substantially insensitive to small variations in the magnitude of the reference current I_(R), the voltage V_(EB) is not insensitive to variations in the temperature of the transistor 20. In particular, the magnitude of the diode reference voltage V_(EB) varies with temperature at about −2 mV/° C., as shown in the graph of FIG. 3. Circuits have therefore been developed to temperature compensate the diode reference voltage V_(EB) shown in FIG. 1. An example of a temperature compensated diode voltage generator 30 is shown in FIG. 4. The reference voltage generator 30 uses a summing amplifier 34 to sum V_(EB) from the PN junction diode of FIG. 1 with a thermal reference voltage V_(T) generated by a thermal voltage source 36 to produce a reference voltage V_(R) at its output. The thermal reference voltage V_(T) is generated from a voltage generated by the thermal voltage source 36 after being boosted by a factor of K using an amplifier 38 having a gain of K.

The thermal reference voltage V_(T) is 0.026V varying with temperature at 0.085 mV/° C. After this value is adjusted by a constant K to make the thermal voltage equal to 0.6 V, a temperature sensitivity of 1.96 mV/° C. is obtained (i.e., K=0.06 V/0.026 V). The sum of the 1.96 mV/° C. thermal sensitivity of the thermal reference voltage V_(T) and the −2 mV/° C. thermal sensitivity of the diode forward voltage V_(EB) results in a terminal sensitivity of the reference voltage V_(R) of only about −0.04 mV/° C., which is substantially insensitive to temperature variations. A graph of the reference voltage V_(R), i.e., the sum of the thermal reference voltage V_(T) and the diode forward voltage V_(EB), is shown in FIG. 6.

Although the temperature compensated diode voltage generator 30 can provide a precise reference voltage that is substantially insensitive to temperature, it is not without its limitations. In particular, as is apparent from FIG. 6, the diode voltage generator 30 generates a reference voltage V_(R) of about 1.25 volts, which inherently requires a supply voltage of at least 1.25 volts. However, electronic devices are increasingly being powered by supply voltages of less than 1.25 volts, thus making the temperature compensated diode voltage generator 30 unsuitable for use in such devices. As a result, there is no relatively simple and inexpensive means to provide a precise, temperature compensated reference voltage in low voltage devices.

There is therefore a need for a method and system for generating a precise reference voltage that is substantially insensitive to temperature and that can be powered by a relatively low supply voltage.

SUMMARY OF THE INVENTION

A reference voltage generator includes a diode voltage generator producing a diode forward V_(EB) voltage, and a thermal voltage generator generating a thermal voltage. The diode forward voltage V_(EB) has a magnitude that has a first predetermined temperature sensitivity. Similarly, the thermal voltage generated by the thermal voltage generator has a magnitude that has a second predetermined temperature sensitivity. A signal combiner receives the diode forward voltage from the diode voltage generator and the thermal voltage from the thermal voltage generator. The signal combiner combines the diode forward voltage with the thermal voltage to provide a reference voltage in a manner that causes the second temperature sensitivity to substantially counteract the first temperature sensitivity. As a result, the reference voltage is substantially insensitive to temperature. The signal combiner combines the diode forward voltage with the thermal voltage without generating any voltage having a magnitude that is substantially greater than either the diode forward voltage or the thermal voltage. As a result, the temperature compensated diode voltage generator is particularly suitable for low supply voltage applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional diode voltage generator.

FIG. 2 is a graph showing the current-voltage characteristics of the diode voltage generator of FIG. 1.

FIG. 3 is a graph showing the magnitude of the diode forward voltage generated by the diode voltage generator of FIG. 1 as a function of temperature.

FIG. 4 is a block diagram of a conventional temperature compensated diode voltage generator.

FIG. 5 is a graph showing the magnitude of a thermal reference voltage generated by a thermal voltage generator in the voltage generator of FIG. 4 as a function of temperature.

FIG. 6 is a graph showing the magnitude of the reference voltage generated by the temperature compensated diode voltage generator of FIG. 4 as a function of temperature.

FIG. 7 is a block diagram of a low voltage, temperature compensated diode voltage generator according to one embodiment of the invention.

FIG. 8 is a schematic diagram of an embodiment of the low voltage, temperature compensated diode voltage generator of FIG. 7.

FIG. 9 is a block diagram of a low voltage, temperature compensated diode voltage generator according to another embodiment of the invention.

FIG. 10 is a block diagram of a memory device using one or more temperature compensated diode voltage generators according to various embodiments of the present invention.

FIG. 11 is a block diagram of a computer system using the memory device of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

A low voltage, temperature compensated diode voltage generator 40 according to one embodiment of the invention is shown in FIG. 7. The reference voltage generator 40 includes the diode voltage generator 10 of FIG. 1 or some other presently known or hereinafter developed diode voltage generator, which generates a diode forward voltage. The diode voltage generator 40 also includes the thermal voltage source 36, which may be any presently known or hereinafter developed thermal voltage generator. The thermal voltage source 36 generates a voltage V_(T), which is amplified by a factor of K by the amplifier 38 to produce a thermal reference voltage of KV_(T). The diode forward voltage V_(EB) generated by the diode voltage generator 10 and the thermal reference voltage KV_(T) generated by the thermal voltage source 36 and amplifier 38 are applied to an averaging circuit 50. As mentioned above, the average of these two voltages is obtained in the prior art by summing the voltages and dividing the summed voltage by two. However, the averaging circuit 50 works in a substantially different manner. Specifically, the averaging circuit 50 directly obtains the average of the two voltages V_(BE) and KV_(T) without simply summing the voltages. More specifically, the averaging circuit 50 preferably obtains the average of the two voltages V_(BE) and KV_(T) without generating any voltage that is significantly greater than either the diode forward voltage V_(BE) or the thermal reference voltage KV_(T). As a result, it is possible (but not necessary) to power the temperature compensated diode voltage generator 40 with a supply voltage having a magnitude that is substantially equal to the diode forward voltage V_(EB). The temperature compensated diode voltage generator 40 is therefore ideally suited to low voltage applications, and, because of its low supply voltage capabilities, may even consume less power compared to prior art temperature compensated diode voltage generators.

A more specific embodiment 60 of the temperature compensated diode reference voltage generator 40 is shown in FIG. 8. The temperature compensated diode voltage generator 60 includes a first current path 62 formed by a PMOS transistor 64 coupled in series with a diode-coupled PNP bipolar transistor 66 between a supply voltage V_(CC) and ground. The transistors 64, 66 set the magnitude of a current I₁ through the current path 62 at a predetermined value.

A second current path 70 is formed by a PMOS transistor 74 coupled in series with a resistor 76 and another diode-coupled PNP bipolar transistor 78 between the supply voltage V_(CC) and ground. A current of I₂ flows through the second current path 70.

A voltage V₁ generated at the junction between the transistors 64, 66 in the first current path 62 and a voltage V₂ generated at the transistor 74 and resistor 76 in the second current path 70 are coupled to respective inputs of a differential amplifier 80 formed by a pair of NMOS transistors 84, 86 in which the gates of the transistors 84, 86 serve as the inputs to the differential amplifier 80. The transistors 84, 86 have their sources connected to each other and to an NMOS current sink transistor 88. The gate of the transistor 88 receives a bias voltage from a diode coupled NMOS transistor 90, which is supplied with current through a resistor 94, thereby setting the current flowing through the transistor 88. The drains of the transistors 84, 86 are coupled to respective PMOS transistors 100, 102, which are connected in a current mirror to ensure that the currents through the transistors 84, 86 are equal to each other. An output voltage V_(O) from the differential amplifier 80 is developed at the drain of the transistor 86, and it is coupled to the gate of the PMOS transistor 74 in the second current path 70 to control the flow of current through the second current path 70.

In operation, the differential amplifier 80 uses the voltage V₁ from the first current path 62 and the voltage V₂ from the second current path 70 to control the current I₂ through the second current path 70 so that it is equal to the current I₁ in the first current path 62. More specifically, if the current I₁ increases, the voltage V₁ will also increase. The resistance of the transistor 86 will then decrease because of the higher voltage applied to the gate of the transistor 86. As a result, the voltage V_(O) will decrease because, as explained above, the current through the transistor 86 will remain equal to the current through the transistor 100 because of the current mirror configuration of the transistors 100, 102. The decreased output voltage V_(O) turns ON the PMOS transistor 74 to a greater extent, thereby increasing the current I₂ so that it is now equal to the current I₁. Conversely, if the current I₁ decreases, the voltage V₁ will also decrease to increase the resistance of the transistor 86. As a result, the output voltage V_(O) will increase, which will reduce the current I₂ so that it is equal to the current I₁.

In a similar manner, if the current I₂ increases, the voltage V₂ will also increase, thereby turning ON the transistor 84 to a greater extent. As a result, the current through the transistors 84, 100 will increase, and, because of the current mirror configuration of the transistors 100, 102, the current through the transistor 102 will also increase. As a result, the voltage V_(O) will also increase, thereby decreasing the current I₂ flowing through the transistor 74. The current I₂ is thus decreased so that it is again equal to the current I₁. Conversely, if the current I₂ decreases, the voltage V₂ will also decrease, thereby decreasing the current flowing through the transistors 84, 100. The current flowing through the transistor 102 will then also decrease to decrease the voltage V_(O), which will increase the current I₂ so that it is again equal to the current I₁. The current I₁ through the first current path 62 will therefore always be equal to the current I₂ flowing through the second current path 70.

It should also be noted that the magnitude of the supply voltage V_(CC) can be just slightly greater than the base/emitter voltage of the transistor 66. A relatively small supply voltage can therefore be used to supply the current flowing in the first and second current paths 62, 70, respectively.

The temperature compensated diode voltage generator 60 also includes a third current path 110 formed by a PMOS transistor 114 and another bipolar PNP transistor 116 coupled between V_(CC) and ground. A current I₃ flows through the third current path 110. The transistor 114 has the same source voltage and gate voltage as the transistors 64, 74. Furthermore, the drain of the transistor 64 is coupled to ground through the bipolar transistor 66, which is identical to the bipolar transistor 116. As a result, the current I₃ is equal to the current I₁ and the current I₂. A diode forward voltage V_(EB) is developed across the transistor 116.

The temperature compensated diode voltage generator 60 also includes a fourth current path 120 through which a fourth current I₄ flows. The fourth current path 120 is formed by a PMOS transistor 124 coupled in series with a resistor 126 between V_(CC) and ground. The current I₄ is equal to the currents I₁, I₂ and I₃ for the same reason that the current I₃ is equal to the currents I₁ and I₂, as explained above. As is well known in the art, a thermal voltage V_(T) is developed across the resistor 126. The resistance of the resistor 126 controls the constant “K” that sets the magnitude of the thermal voltage KV_(T) so that it is equal to the magnitude of the diode forward voltage V_(BE) at a specific temperature. Of course several resistors can be used in place of the set of the single resistor 126. In an integrated circuit implementation, the use of several resistors rather than the single resistor 126 shown in FIG. 8 would allow the resistance to be varied by varying the number of resistors.

The diode forward voltage V_(BE) and the thermal voltage KV_(T) are applied to an averaging circuit 130 formed by a first averaging resistor 132 coupled to an output node 134 and receiving the diode forward voltage V_(EB), and a second averaging resistor 138 coupled to the output node 134 and receiving the thermal voltage KV_(T). A thermally compensated reference voltage V_(R) is developed at the output node 134, and is filtered by an NMOS transistor 140 connected to the output node 134 to form a capacitor. The resistors 132, 138 form a voltage divider so that the magnitude of the reference voltage V_(R) is given by the formula: V_(R)=(V_(EB)+KV_(T))/2, which is the average of the diode forward voltage V_(BE) and the thermal voltage KV_(T). It should be noted, however, that the average is obtained without summing the diode forward voltage V_(BE) and the thermal voltage KV_(T), which would require a voltage substantially larger than either the diode forward voltage V_(EB) or the thermal voltage KV_(T). As a result, the temperature compensated diode voltage generator 60 can generate a temperature compensated diode forward voltage V_(R) using a supply voltage V_(CC) having a magnitude that is only slightly greater than the magnitude of the diode forward voltage V_(BG).

It should be noted that one embodiment of a prior art reference voltage generator uses a circuit that is similar to the circuit shown in FIG. 8 except that it connects the resistor 126 to ground through a diode coupled bipolar PNP transistor (not shown), which generates a diode forward voltage. As a result, the prior art circuit inherently sums the diode forward voltage V_(EB) with the thermal voltage KV_(T), thus requiring a supply voltage V_(CC) having a magnitude that is greater than the magnitude of V_(BE)+KV_(T).

Another embodiment of a temperature compensated diode voltage generator 140 is shown in FIG. 9. The diode voltage generator 140 also includes the diode voltage generator 10 of FIG. 1 (or some other presently known or hereinafter developed diode voltage generator) as well as the thermal voltage source 36 (which may also be any presently known or hereinafter developed thermal voltage generator). The diode forward voltage V_(EB) generated by the diode voltage generator 10 is scaled by a factor of ½ using a first attenuation circuit 144 to produce a voltage of V_(BE)/2. The voltage V_(T) generated by the thermal voltage source 36 is scaled by a factor of K/2 using an attenuation circuit 146 to produce a thermal reference voltage KV_(T)/2. These two voltages are then applied to a summer 150, which produces a temperature compensated voltage V_(R) equal to the average of the diode forward voltage V_(EB) and the thermal voltage KV_(T). Note that the voltage V_(R) can again be obtained using a supply voltage V_(CC) having a magnitude that is only slightly greater than the magnitude of the diode forward voltage V_(EB) and the magnitude of the thermal voltage KV_(T).

A temperature compensated diode voltage generator can be advantageously used in a memory device, such a synchronous dynamic random access memory (“SDRAM”) 200 shown in FIG. 10, various embodiments of the invention may also be used in other types of memory devices and in electronic circuits other than memory devices. The SDRAM 200 includes an address register 212 that receives either a row address or a column address on an address bus 214. The address bus 214 is generally coupled to a memory controller (not shown). Typically, a row address is initially received by the address register 212 and applied to a row address multiplexer 218. The row address multiplexer 218 couples the row address to a number of components associated with either of two memory banks 220, 222 depending upon the state of a bank address bit forming part of the row address. Associated with each of the memory banks 220, 222 is a respective row address latch 226, which stores the row address, and a row decoder 228, which applies various signals to its respective array 220 or 222 as a function of the stored row address. The row address multiplexer 218 also couples row addresses to the row address latches 226 for the purpose of refreshing the memory cells in the arrays 220, 222. The row addresses are generated for refresh purposes by a refresh counter 230, which is controlled by a refresh controller 232.

After the row address has been applied to the address register 212 and stored in one of the row address latches 226, a column address is applied to the address register 212. The address register 212 couples the column address to a column address latch 240. Depending on the operating mode of the SDRAM 200, the column address is either coupled through a burst counter 242 to a column address buffer 244, or to the burst counter 242 which applies a sequence of column addresses to the column address buffer 244 starting at the column address output by the address register 212. In either case, the column address buffer 244 applies a column address to a column decoder 248 which applies various signals to respective sense amplifiers and associated column circuitry 250, 252 for the respective arrays 220, 222.

Data to be read from one of the arrays 220, 222 is coupled to the column circuitry 250, 252 for one of the arrays 220, 222, respectively. The data is then coupled through a read data path 254 to a data output register 256, which applies the data to a data bus 258. Data to be written to one of the arrays 220, 222 is coupled from the data bus 258, a data input register 260 and a write data path 262 to the column circuitry 250, 252 where it is transferred to one of the arrays 220, 222, respectively. A mask register 264 may be used to selectively alter the flow of data into and out of the column circuitry 250, 252, such as by selectively masking data to be read from the arrays 220, 222.

The above-described operation of the SDRAM 200 is controlled by a command decoder 268 responsive to command signals received on a command bus 270. These high level command signals, which are typically generated by a memory controller (not shown), are a clock enable signal CKE*, a clock signal CLK, a chip select signal CS*, a write enable signal WE*, a row address strobe signal RAS*, and a column address strobe signal CAS*, which the “*” designating the signal as active low. Various combinations of these signals are registered as respective commands, such as a read command or a write command. The command decoder 268 generates a sequence of control signals responsive to the command signals to carry out the function (e.g., a read or a write) designated by each of the command signals. These command signals, and the manner in which they accomplish their respective functions, are conventional. Therefore, in the interest of brevity, a further explanation of these control signals will be omitted.

The SDRAM 200 further includes a charge pump 290 supplying a voltage to various components in the SDRAM 200. The magnitude of the voltage supplied by the charge pump 290 is controlled by a reference voltage V_(R) generated by a temperature compensated diode voltage generator 294 in accordance with the present invention. One or more of the temperature compensated diode voltage generators 294 can also be used to supply reference voltages for other purposes, and the temperature compensated diode voltage generator can be used in devices other than memory devices.

A computer system 300 containing the SDRAM 200 of FIG. 11. The computer system 300 includes a processor 302 for performing various computing functions, such as executing specific software to perform specific calculations or tasks. The processor 302 includes a processor bus 304 that normally includes an address bus, a control bus, and a data bus. In addition, the computer system 300 includes one or more input devices 314, such as a keyboard or a mouse, coupled to the processor 302 to allow an operator to interface with the computer system 300. Typically, the computer system 300 also includes one or more output devices 316 coupled to the processor 302, such output devices typically being a printer or a video terminal. One or more data storage devices 318 are also typically coupled to the processor 302 to allow the processor 302 to store data in or retrieve data from internal or external storage media (not shown). Examples of typical storage devices 318 include hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). The processor 302 is also typically coupled to cache memory 326, which is usually static random access memory (“SRAM”), and to the SDRAM 200 through a memory controller 330. The memory controller 330 normally includes the control bus 270 and the address bus 214 that are coupled to the SDRAM 200. The data bus 258 is coupled from the SDRAM 200 to the processor bus 304 either directly (as shown), through the memory controller 330, or by some other means.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, it will be understood by one skilled in the art that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. 

1. A reference voltage generator comprising: a first voltage generator configured to generate a first voltage, the first voltage generator comprising: a bipolar transistor having a emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source; and a first current source coupled between the emitter of the bipolar transistor and a second voltage source, the first current source causing a first current to flow between the emitter and collector of the bipolar transistor to cause the first voltage to be generated at the emitter of the bipolar transistor; a second voltage generator configured to generate a second voltage; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage and the second voltage, the averaging circuit comprising: a first resistor coupled between an output node and the first voltage generator so that the first voltage is coupled to the output node through the first resistor; and a second resistor coupled between the output node and the second voltage generator so that the second voltage is coupled to the output node through the second resistor.
 2. The reference voltage generator of claim 1 wherein the resistance of the first resistor is equal to the resistance of the second resistor.
 3. The reference voltage generator of claim 1 wherein the averaging circuit comprises a voltage divider formed by coupling the output node between the first resistor and the second resistor, wherein the output node provides the reference voltage.
 4. The reference voltage generator of claim 1 wherein the first voltage source comprises a ground potential.
 5. The reference voltage generator of claim 1 wherein the second voltage generator comprises: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the second terminal of the resistor.
 6. The reference voltage generator of claim 5 wherein the magnitude of the first current is substantially equal to the magnitude of the second current.
 7. The reference voltage generator of claim 5 wherein the averaging circuit comprises: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging resistor.
 8. The reference voltage generator of claim 7 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage.
 9. A supply voltage circuit comprising: a charge pump generating a supply voltage having a magnitude determined by a reference voltage applied to a reference voltage input; and a reference voltage generator having an output node coupled to the reference voltage input of the charge pump, the reference voltage generator comprising: a first voltage generator configured to generate a first voltage, the first voltage generator comprising: a bipolar transistor having an emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source; and a first current source coupled between the emitter of the bipolar transistor and a second voltage source, the first current source causing a first current to flow between the emitter and collector of the bipolar transistor to cause the first voltaae to be generated at the emitter of the bipolar transistor; a second voltage generator configured to generate a second voltage; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage and the second voltage, the averaging circuit comprising: a first resistor coupled between an output node and the first voltage generator so that the first voltage is coupled to the output node through the first resistor; and a second resistor coupled between the output node and the second voltage generator so that the second voltage is coupled to the output node through the second resistor.
 10. The supply voltage circuit of claim 9 wherein the resistance of the first resistor is equal to the resistance of the second resistor.
 11. The supply voltage circuit of claim 9 wherein the averaging circuit comprises a voltage divider formed by the output node between the first resistor and the second resistor, wherein the output node provides the reference voltage.
 12. The supply voltage circuit of claim 9 wherein the first voltage source comprises a ground potential.
 13. The supply voltage circuit of claim 9 wherein the second voltage generator comprises: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the second terminal of the resistor.
 14. The supply voltage circuit of claim 13 wherein the magnitude of the first current is substantially equal to the magnitude of the second current.
 15. The supply voltage circuit of claim 14 wherein the averaging circuit comprises: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging resistor.
 16. The supply voltage circuit of claim 15 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage.
 17. A method of generating a reference voltage, comprising: generating a first voltage signal by coupling a first current through a bipolar transistor having an emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source and generating the first voltage signal at the emitter of the bipolar transistor; generating a second voltage signal; and averaging the first voltage signal and the second voltage signal without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage signal and the second voltage signal, the act of averaging comprising coupling the first voltage signal and the second voltage signal to an output node through respective impedance elements.
 18. The method of claim 17 wherein the act of averaging the first voltage signal with the second voltage signal comprises forming a voltage divider by coupling the output node between a first impedance element and a second impedance element, wherein the reference voltage is provided by the output node.
 19. The method of claim 17 wherein the first voltage signal comprises a forward junction voltage.
 20. The method of claim 17 wherein the first voltage source comprises a ground potential.
 21. The method of claim 17 wherein the act of generating the second voltage signal comprises coupling a first current through a resistor, the second voltage signal being generated across the resistor.
 22. The method of claim 21 wherein the second voltage signal comprises a thermal voltage.
 23. A reference voltage generator comprising: a first voltage generator configured to generate a first voltage; a second voltage generator configured to generate a second voltage; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is significantly greater than the greater of the first voltage and the second voltage, the averaging circuit comprising: a first resistor coupled between an output node and the first voltage generator so that the first voltage is coupled to the output node through the first resistor; and a second resistor coupled between the output node and the second voltage generator so that the second voltage is coupled to the output node through the second resistor, the first resistor and the second resistor forming a voltaae divider with the output node being between the first resistor and the second resistor, the output node providing the reference voltage.
 24. The reference voltage generator of claim 23 wherein the resistance of the first resistor is equal to the resistance of the second resistor.
 25. A supply voltage circuit comprising: a charge pump generating a supply voltage having a magnitude determined by a reference voltage applied to a reference voltage input; and a reference voltage generator having an output node coupled to the reference voltage input of the charge pump, the reference voltage generator comprising: a first voltage generator configured to generate a first voltage, the first voltage generator comprising: a bipolar transistor having a emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source; and a first current source coupled between the emitter of the bipolar transistor and a second voltage source, the first current source causing a first current to flow between the emitter and collector of the bipolar transistor to cause the first voltage to be generated at the emitter of the bipolar transistor; a second voltage generator configured to generate a second voltage, the second voltage generator comprising: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the second terminal of the resistor; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is significantly greater than the greater of the first voltage and the second voltage.
 26. The supply voltage circuit of claim 25 wherein the first voltage source comprises a ground potential.
 27. The supply voltage circuit of claim 25 wherein the magnitude of the first current is substantially equal to the magnitude of the second current.
 28. The supply voltage circuit of claim 27 wherein the averaging circuit comprises: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging register.
 29. The supply voltage circuit of claim 28 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage.
 30. A reference voltage generator comprising: a first voltage generator configured to generate a first voltage, the first voltage generator comprising: a bipolar transistor having a emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source; and a first current source coupled between the emitter of the bipolar transistor and a second voltage source, the first current source causing a first current to flow between the emitter and collector of the bipolar transistor to cause the first voltage to be generated at the emitter of the bipolar transistor; a second voltage generator configured to generate a second voltage, the second voltage generator comprising: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the second terminal of the resistor; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage and the second voltage.
 31. The reference voltage generator of claim 30 wherein the second voltage generator comprises: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the second terminal of the resistor.
 32. The reference voltage generator of claim 30 wherein the magnitude of the first current is substantially equal to the magnitude of the second current.
 33. The reference voltage generator of claim 30 wherein the averaging circuit comprises: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging resistor.
 34. The reference voltage generator of claim 33 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage.
 35. A supply voltage circuit comprising: a charge pump generating a supply voltage having a magnitude determined by a reference voltage applied to a reference voltage input; and a reference voltage generator having an output node coupled to the reference voltage input of the charge pump, the reference voltage generator comprising: a first voltage generator configured to generate a first voltage; a second voltage generator configured to generate a second voltage, the second voltage generator comprising: a resistor having first and second terminals, the first terminal of the resistor being coupled to the first voltage source; and a second current source coupled between the second terminal of the resistor and the second voltage source, the second current source causing a second current to flow through the resistor to cause the second voltage to be generated at the, second terminal of the resistor; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage and the second voltage, the averaging circuit comprising: a first resistor coupled between an output node and the first voltage generator so that the first voltage is coupled to the output node through the first resistor; and a second resistor coupled between the output node and the second voltage generator so that the second voltage is coupled to the output node through the second resistor.
 36. The supply voltage circuit of claim 35 wherein the magnitude of the first current is substantially equal to the magnitude of the second current.
 37. The supply voltage circuit of claim 36 wherein the averaging circuit comprises: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging resistor.
 38. The supply voltage circuit of claim 37 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage.
 39. A method of generating a reference voltage, comprising: generating a first voltage signal by coupling a first current through a bipolar transistor having an emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source and generating the first voltage signal at the emitter of the bipolar transistor; generating a second voltage signal by coupling a first current through a resistor, the second voltage signal being generated across the resistor; and averaging the first voltage signal and the second voltage signal without generating any voltage having a magnitude that is substantially greater than the greater of the first voltage signal and the second voltage signal.
 40. The method of claim 39 wherein the second voltage signal comprises a thermal voltage.
 41. A supply voltage circuit comprising: a charge pump generating a supply voltage having a magnitude determined by a reference voltage applied to a reference voltage input; and a reference voltage generator having an output node coupled to the reference voltage input of the charge pump, the reference voltage generator comprising: a first voltage generator configured to generate a first voltage, the first voltage generator comprising: a bipolar transistor having a emitter, a base and a collector, the bipolar transistor having its base and collector coupled to each other and to a first voltage source; and a first current source coupled between the emitter of the bipolar transistor and a second voltage source, the first current source causing a first current to flow between the emitter and collector of the bipolar transistor to cause the first voltage to be generated at the emitter of the bipolar transistor; a second voltage generator configured to generate a second voltage; and an averaging circuit coupled to receive the first voltage from the first voltage generator and the second voltage from the second voltage generator, the averaging circuit operable to generate a reference voltage by averaging the first voltage and the second voltage without generating any voltage having a magnitude that is significantly greater than the greater of the first voltage and the second voltage, the averaging circuit comprising: a first averaging resistor coupled between the emitter of the bipolar transistor and the output node to couple the first voltage generator to the output node through the first averaging resistor; and a second averaging resistor coupled between the second terminal of the resistor and the output node to couple the second voltage generator to the output node through the second averaging resistor.
 42. The supply voltage circuit of claim 41 wherein the first voltage generator is configured to provide a forward junction voltage and the second voltage generator is configured to provide a thermal voltage. 